Traditionally, as a method for immunoassay of biomaterials, fluoroimmunoassay has been conducted using an optical fiber with an immunological substance such as an antigen or antibody immobilized thereto.
In this case, to facilitate the immobilization of an immunological substance on the surface of the optical fiber, a crosslinking agent highly reactive with the amino group in the antigen or antibody is introduced on the surface of the optical fiber. For example, WO90/13029 proposes a method wherein an optical fiber is reacted with a dialdehyde such as glutaraldehyde or succinaldehyde to introduce formyl groups on a core surface thereof, thereby facilitating immobilization of an immunological substance on the surface of the optical fiber, and the formyl groups are reacted and bound with the amino group in a bioactive component such as an antigen or antibody to immobilize the immunological substance to the optical fiber.
Specifically, in the method of WO90/13029, a plastic optical fiber mainly composed of a resin having an ester structure is used as an immobilization carrier for immunological substances such as bioactive components, and formyl groups are introduced on a core surface of the plastic optical fiber by carrying out the reaction while immersing the core surface of the plastic optical fiber in a mixture of a 50 to 100 mM KOH solution in ethanol solvent, an NiSO.sub.4 solution in ethanol solvent and glutaraldehyde or succinaldehyde. The formyl groups thus introduced on the core surface are then bound to the amino group in the immunological substance to prepare a plastic optical fiber with the immunological substance immobilized thereto.
However, in the method of WO90/13029, since a dialdehyde is used to carry out the reaction in introducing formyl groups on a core surface of the optical fiber, the optical fiber is likely to have a polymeric structure formed thereon. As a result, the core surface of the optical fiber tends to have turbidity. A problem has been pointed out that this turbidity results in increased transmission losses of fluorescence when the optical fiber is used for fluoroimmunoassay, etc.